The amount of acidic crude oil produced around the globe is increasing as more sources of oil are found. Acidic crude oils contain significant amounts of carboxylic acids and are characterised as high-TAN crudes (total acid number). In the early phase of production, most wells produce oils with low water contents, typically below 0.5 wt %. As time goes by, water break-through occurs giving rise to significantly higher water cuts. This means that the carboxylic acids present in high TAN crudes will be in intimate contact with water during production.
These high TAN oils introduce problems throughout the hydrocarbon value chain due to their high content of carboxylic acids. Of greatest concern are the formation of naphthenate deposits which occur when the crude oil is brought to the surface in the presence of water. These deposits can block all manner of equipment which can lead to process irregularities and potentially production shutdown.
Calcium naphthenate deposition is among the most challenging obstacles to high production regularity in oil fields where acidic crudes are produced. Until a few years ago, it was believed that the naphthenate deposits formed during crude oil production were made up of calcium soaps of naphthenic acids in the crude oil with a slight over-representation of lighter acids. More recent research has shown this to be incorrect.
The real dominating materials are the salts of the so called “ARN” acids, a family of 4-protic carboxylic acids containing 4 to 8 rings in the hydrocarbon skeleton with molecular weights in the range 1227 to 1235 g/mol. The most prominent among these acids exhibits 6 sites of unsaturation (5-member rings) and a molecular weight of 1231; its chemical formula being C80H142O8. The other members of the ARN family either have different numbers of rings and/or additional CH2-groups in the hydrocarbon skeleton. This discovery was reported in Prepr. Pap-Am Chem Soc Div Pet Chem 2004, 49(3), 274 and SPE 93011, SPE 5th Int. Oilfield scale symposium Aberdeen 2005 and has had some fundamental effects on the way in which high TAN crudes are treated. ARN is not an abbreviation but is simply the term which has been coined to define the acids in question.
The mechanism by which the ARN acid salts form is not fully understood but it is envisaged that the calcium salts form as the crude oil is produced from the oil well. It is believed that at the pressures and pH present in the oil reservoir, the calcium salt normally does not form. Moreover, the salt is not formed during production if no water is coproduced because there are essentially no metal ions to form the ARN acid salts. Where the crude is coproduced with water however, e.g. in a field where water breakthrough has occurred, as the material comes to the surface pressure reduces and pH increases due to carbon dioxide release. Under higher pH conditions, calcium ions from water (or any other suitable metal ion) can react with the ARN carboxylic acids naturally present in the crude oil to form calcium salts of the ARN acids, i.e. naphthenates. It is envisaged that this reaction takes place when a pH of around 5-5.5 is reached.
The ARN salt reaction product is essentially insoluble in both oil and water and hence precipitates out in whatever vessel the crude oil is produced into, typically a separator for removing water. The ARN salts accumulate at the oil and water interface and the presence of this material is a major problem for the oil producer. It is a highly viscous semi solid material that can block filters, nozzles, instrument outtakes, pumps and pipes in oilfield machinery. The industry therefore takes steps to suppress the formation of the ARN acid salts, either by pH-regulation or chemical injection.
In some cases, suppression of the ARN salt formation can be achieved by adding a water soluble acid to a well with a high water cut. Acid injection reverses ARN salt formation as increasing pH is a prerequisite for its formation.
This solution is expensive however and may cause long term corrosion of equipment so proprietary ARN acid salt inhibitors are now used to prevent salt formation in high TAN crude oil refining. These inhibitors are constantly refined, e.g. to take account of the changing nature of the crude being extracted, but have proven extremely effective. In the applicant's oil field at Heidrum, their use has prevented any loss of production for over 10 years. The problem therefore of ARN acid salt formation is one which the industry has overcome successfully by preventing them ever coming into existence.